Every $30 purchased?

Just stumbled upon a funny ad. It says "Every $30 purchased (except for cigarettes and liquor) entitles you to purchase any purchase with purchase items. (Limited to 1 piece of every item for each customer) While stocks last. No traders allowed."

Firstly, you don't purchase $30 right? And this, "entitles you to purchase any purchase with purchase...", it is confusing isn't it?

If I patronise this shop with 20 over friends' and relatives' company, could we get over 20 pcs of each item? Lastly, help! What do they mean by traders?

Perhaps this is a better way to communicate to their customers:
"Purchase with purchase items: Valid with every $30 worth of purchases(excluding cigarettes and liquor sales). While stocks last. Limited to 1 piece per item for every receipt."

Did I missed out or committed more errors? :x